Computational Modelling of Cationic Contamination in PEFCs: a Multiphase Mixture Approach

Abstract

Cationic contamination in polymer electrolyte membrane fuel cells (PEFCs) can lead to accelerated degradation and loss of performance. There are several mechanisms to introduce these cationic impurities; including transport with the inlet fuel/air streams, or from degradation of the fuel cell and balance of plant components. Many common cations tend to have a higher affinity to the sulfonic acid side chains of the polymer electrolyte membrane (PEM) than protons, resulting in preferential uptake of these contaminants into the PEM. Once occupying these sites, the impurity cations contribute to the loss of performance by lowering the water content of the membrane and reducing the ionic conductivity.[1] To compliment experimental results, numerical studies have been performed[2,3] to understand the coupled impact of cation occupancy in the membrane with the water transport in the cell and the overall cell performance. Early generations of these models had assumed a constant cation occupancy in the cathode that was used as a boundary condition to solve the cation transport via a Stefan-Maxwell diffusion process. By coupling the contaminant occupancy to the water content of the membrane and electro-kinetics of the catalyst layers, cell performance has been shown to decrease down to as little as one third its non-contaminated levels.[3] This study builds off of previous models to develop a steady state, one dimensional, cation contamination model of a PEFC to examine the impact of cation transport to the membrane electrode assembly (MEA) via a water bridge across the gas diffusion layer (GDL) as shown in Figure 1. The multiphase mixture (M2) model, proposed by Wang and Cheng[4], is used to derive species transport equations for the reactant gases and water that permits two phase water transport in the GDL and catalyst layers. This resulting water distribution acts as to promote cation transport from the gas flow channels across the GDL, where the dissolved cations can evolve out of the liquid phase water and into the MEA, occupying the sulfonic acid sites of the membrane. A baseline non-contaminated case is presented to study the cell performance and species distribution normal operating conditions. Then, the model is employed to show how the saturation of the cathode effects cation distribution across the GDL and how the presence of dissolved cations in the catalyst layer impacts the contaminant occupancy and overall cell performance.